Fix various typos, man page issues and improve the documentation (#415)

This commit makes various different improvements to the documentation:
- sh.1: Backported (with changes) mandoc warning fixes from ksh2020
  for the ksh93(1) man page: <https://github.com/att/ast/pull/1406>
- Removed unnecessary spaces at the end of lines to fix a few other
  mandoc warnings.
- Fixed various typos and capitalization errors in the documentation.
- ANNOUNCE: Document the addition of the ${.sh.pid} variable
  (re: 9de65210).
- libast/man/str*: Update the man pages for the libast str* functions
  to improve how accurately each function is described.
- ksh93/README: Update regression test/compatibility notes to include
  OpenBSD 7.0, FreeBSD 13.0 and WSL running Ubuntu 20.04.
- Change a few places to store the return value from strlen in a
  size_t variable rather than signed int.
- comp/setlocale.c: To avoid confusion of two separate variables named
  lang, the function local variable has been renamed to langidx.

src/lib/libast/sfio/sfmode.c

@@ -147,7 +147,7 @@ int _sfsetpool(Sfio_t* f)
 	}
 
 	/* always add at end of array because if this was done during some sort
-	   of walk thru all streams, we'll want the new stream to be seen.
+	   of walk through all streams, we'll want the new stream to be seen.
 	*/
 	p->sf[p->n_sf++] = f;
 	rv = 0;